Although fire is now rarely used in synthetic chemistry, it was not until Robert Bunsen invented the burner in 1855 that the energy from this heat source could be applied to a reaction vessel in a focused manner. The Bunsen burner was later superseded by the isomantle, oil bath, or hot plate as a source for applying heat to a chemical reaction. In the past few years, heating and driving chemical reactions by microwave energy has been an increasingly popular theme in the scientific community. This nonclassical heating technique is slowly moving from a laboratory curiosity to an established technique that is heavily used in both academia and industry. The efficiency of "microwave flash heating" in dramatically reducing reaction times (from days and hours to minutes and seconds) is just one of the many advantages. This Review highlights recent applications of controlled microwave heating in modern organic synthesis, and discusses some of the underlying phenomena and issues involved.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/anie.200400655

Controlled microwave heating in modern organic synthesis.

Since the discovery of organic azides by Peter Griess more than 140 years ago, numerous syntheses of these energy-rich molecules have been developed. In more recent times in particular, completely new perspectives have been developed for their use in peptide chemistry, combinatorial chemistry, and heterocyclic synthesis. Organic azides have assumed an important position at the interface between chemistry, biology, medicine, and materials science. In this Review, the fundamental characteristics of azide chemistry and current developments are presented. The focus will be placed on cycloadditions (Huisgen reaction), aza ylide chemistry, and the synthesis of heterocycles. Further reactions such as the aza-Wittig reaction, the Sundberg rearrangement, the Staudinger ligation, the Boyer and Boyer-Aube rearrangements, the Curtius rearrangement, the Schmidt rearrangement, and the Hemetsberger rearrangement bear witness to the versatility of modern azide chemistry.

Organic Azides: An Exploding Diversity of a Unique Class of Compounds

Asymmetric 1,3-Dipolar Cycloaddition Reactions

The Diels-Alder reaction has both enabled and shaped the art and science of total synthesis over the last few decades to an extent which, arguably, has yet to be eclipsed by any other transformation in the current synthetic repertoire. With myriad applications of this magnificent pericyclic reaction, often as a crucial element in elegant and programmed cascade sequences facilitating complex molecule construction, the Diels-Alder cycloaddition has afforded numerous and unparalleled solutions to a diverse range of synthetic puzzles provided by nature in the form of natural products. In celebration of the 100th anniversary of Alder's birth, selected examples of the awesome power of the reaction he helped to discover are discussed in this review in the context of total synthesis to illustrate its overall versatility and underscore its vast potential which has yet to be fully realized.

https://www.ch.ic.ac.uk/local/organic/pericyclic/DA.pdf

The Diels--Alder reaction in total synthesis.

Introduction Cationic domino reactions Anionic domino reactions Radical domino reactions Pericyclic domino reactions Photochemically induced domino processes Transition metal catalysis Domino reactions initiated by oxidation or reduction Enzymes in domino reactions Multicomponent reactions Special techniques in domino reactions

Domino Reactions in Organic Synthesis

Benzyne, cyclohexyne, and 3-azacyclohexyne have been obtained by flash vacuum pyrolysis of phthalic anhydride, 4-cyclopentylidene-3-methylisoxazol-5(4H)-one (34), and 3-methyl-4-(2-pyrrolidinylidene)isoxazol-5(4H)-one (37), respectively, isolated at 77 K and observed by IR spectroscopy. Trapping and fragmentation reactions are described, and the implications for the mechanism of benzyne formation from phthalic anhydride via cyclopentadienylideneketene (9) are discussed.

Benzyne, cyclohexyne, and 3-azacyclohexyne and the problem of cycloalkyne versus cycloalkylideneketene genesis

High-level ab initio molecular orbital calculations, using basis sets up to 6-311+G(2d,2p) with electron correlation incorporated at the quadratic configuration interaction [QCISD(T)] level, have been used to study the tautomeric equilibrium and hydrogen shifts of tetrazole in the gas phase and in solution. The solvent effects were investigated by self-consistent reaction field (SCRF) theory. Consistent with experimental observations, the 1H-tetrazole (1)/2H-tetrazole (2) tautomeric equilibrium is calculated to be strongly influenced by the surrounding medium. 2H-Tetrazole is the energetically preferred tautomer in the gas phase. In a nonpolar solution, both the 1H and 2H forms are predicted to exist in comparable amounts. However, in a medium of high dielectric constant the more polar 1H tautomer is the dominant species. The calculated free energy changes for tautomerization of IH-tetrazole in the gas phase and in nonpolar (epsilon = 2) and polar (epsilon = 40) media are -7, 1, and 12 kJ mol-1, respectively. The molecular geometry, charge distribution, and vibrational frequencies of the polar 1H tautomer are found to be altered significantly in the presence of a solvent reaction field. Isomerization of 1 to 2, via a [1,2] hydrogen shift, requires an energy barrier of 207 kJ mol-1 in the gas phase. 5H-Tetrazole (3) is predicted to lie 82 kJ mol-1 above 1, due to its nonaromatic character. However, rearrangement of 3 to 1, via a [1,5] hydrogen shift, is inhibited by an activation barrier of 150 kJ mol-1. Conversely, the energy barrier for the rearrangement of 1 to 3 is 232 kJ mol-1, slightly larger than that required for the isomerization of 1 to 2. These results suggest that 3 is a good candidate for experimental observation. Inclusion of electron correlation leads to a drastic change in the molecular geometry of 3. At the MP2 level, an acyclic structure is predicted, while at the MP3 and QCISD levels the expected cyclic structure is found, The calculated molecular geometry of 1H-tetrazole at the MP2 level is found to differ significantly from the available solid-state structural data.

Tautomeric equilibrium and hydrogen shifts of tetrazole in the gas phase and in solution

Ab initio calculations at the G2(MP2,SVP) and B-LYP/6-311+G(3df,2p)+ZPVE levels have been used to examine the potential energy surface of C7H6. Fulvenallene (6) is the most stable C7H6 isomer considered in this study. 1-Ethynylcyclopentadiene (9A), benzocyclopropene (10), and 1,2,4,6-cycloheptatetraene (4) lie 12, 29, and 49 kJ mol-1, respectively, above 6. Phenylcarbene (1) is calculated is to have a triplet (3A‘‘) ground state, 16 kJ mol-1 more stable than the singlet state (1A‘). Interconversion of 1 and 4 is predicted to have a moderate activation barrier, with the involvement of a stable bicyclic intermediate (bicyclo[4.1.0]hepta-2,4,6-triene, 2). However, 2 is found to lie in a shallow potential energy well with a small barrier (8 kJ mol-1) to rearrangement to 4. At the G2(RMP2,SVP)//QCI level, the 3A2 and 3B1 triplet states of cycloheptatrienylidene (33) are predicted to lie very close in energy. The singlet “aromatic” cycloheptatrienylidene (13, 1A1) is found to be a transition structure interconv...

Interconversions of Phenylcarbene, Cycloheptatetraene, Fulvenallene, and Benzocyclopropene. A Theoretical Study of the C7H6 Energy Surface

This review describes the methods of generation of alpha-oxoketenes and their use in synthesis. While the ketenes are often generated in situ without direct proof for their existence, methods used for their direct observation are also emphasized. The most important classes of precursor molecules are 2-diazo-1,3-diketones, 1,3-dioxin-4-ones, 2,3-dihydrofuran-2,3-diones, and beta-ketoacid derivatives. Synthetically useful reactions are nucleophilic additions to give carboxylic acid derivatives which can be subjected to further functional group manipulation, [2 + 2] cycloadditions to give four-membered rings, and [2 + 4] cycloaddition chemistry with a wide variety of double-and triple-bonded dienophiles, resulting in numerous six-membered heterocyclic ring systems.

α-Oxoketenes - Preparation and Chemistry

The structures and reactivities of nitrile imines are subjects of continuing debate. Several nitrile imines were generated photochemically or thermally and investigated by IR spectroscopy in Ar matrices at cryogenic temperatures (Ph-CNN-H 6, Ph-CNN-CH3 17, Ph-CNN-SiMe3 23, Ph-CNN-Ph 29, Ph3C-CNN-CPh3 34, and the boryl-CNN-boryl derivative 39). The effect of substituents on the structures and IR absorptions of nitrile imines was investigated computationally at the B3LYP/6-31G* level. IR spectra were analyzed in terms of calculated anharmonic vibrational spectra and were generally in very good agreement with the calculated spectra. Infrared spectra were found to reflect the structures of nitrile imines accurately. Nitrile imines with IR absorptions above 2200 cm–1 have essentially propargylic structures, possessing a CN triple bond (typically PhCNNSiMe3 23, PhCNNPh 29, and boryl-CNN-boryl 39). Nitrile imines with IR absorptions below ca. 2200 cm–1 are more likely to be allenic (e.g., HCNNH 1, PhCNNH 6, HCNN...

Curt Wentrup

Papers

Benzyne, cyclohexyne, and 3-azacyclohexyne and the problem of cycloalkyne versus cycloalkylideneketene genesis

Tautomeric equilibrium and hydrogen shifts of tetrazole in the gas phase and in solution

Interconversions of Phenylcarbene, Cycloheptatetraene, Fulvenallene, and Benzocyclopropene. A Theoretical Study of the C7H6 Energy Surface

α-Oxoketenes - Preparation and Chemistry

Nitrile Imines: Matrix Isolation, IR Spectra, Structures, and Rearrangement to Carbodiimides